Between 1983 and 1995, a total of 32,064 pesticide analyses were done on samples from 4,155 wells in the study area (Matzen and Saad, 1996). In 29 percent of the sampled wells, at least one pesticide was detected. Atrazine and its degradation products exceeded the Wisconsin preventive action limit (PAL) of 0.3 microgram per liter in about 10 percent of all wells sampled. Thousands of potential pesticide degradation products exist and these compounds are rarely included in laboratory analyses for pesticides. Therefore, it is likely that pesticide and degradation product occurrences in both surface and ground water have been, and currently are, underestimated in the study area and across the Nation.